Unrouted handler imposes unnecessary requirement on upload path

[innovate-invent]

Describe the bug The existence of this function imposes an unnecessary restriction on the URI path and software attempting to use the dependent library.

Expected behavior extractIDFromPath() should be replaced with a callback:

type UnroutedHandler struct {
...
   GetID func(r *http.Request)(string, error)
}

GetID can default to the current functionality

Setup details golang tusd handler library

[Acconut]

See the corresponding PR for more discussion: https://github.com/tus/tusd/pull/341